Brush seals are commonly used to eliminate or minimize air leakage through a gap between parts or components that may be positioned adjacent to each other. For example, brush seals are positioned in rotating mechanisms such as turbine engines used for power generation and the like. Typically, the brush seals minimize the leakage between regions at different pressures on opposite sides of the seal. As a specific example, a brush seal may be used to minimize air leakage through the gap (or clearance) between a stationary component such as a stator and a rotating component such as a rotor. Brush seals are contact seals with bristles contacting the rotor surface so as to allow for a tight clearance and leakage reduction as compared to non-contact seals such as labyrinth seals and the like.
The bristles of a brush seal, however, may undergo substantial wear due to interference between the bristles and the rotor caused by thermal transients during, for example, turbine start up or shutdown. This wear may accumulate over a number of startups/shutdown cycles so as to reduce the leakage performance of the seal during steady state operations. Wear in the bristles generally correlates with an overall decrease in turbine efficiency and power output.
A retractable brush seal may eliminate the wear due to thermal interference during startup or shutdown by physically moving the seal away from the rotor. A retractable brush seal may be passively activated by means of leaf springs and the like that respond to the varying pressure differential across the seat. The retractable brush seal may be positioned in a high clearance position such that the increasing pressure deferential during startup deforms the leaf springs to move the seal closer to the rotor. Similarly during shutdown, the falling pressure differential causes the leaf springs to retract so as to move the seal away from the rotor. The retractable brush seal thus eliminates or reduces bristle/rotor interference so as to provide an increased component lifetime.
The retractable brush seal usually is mounted in a mating slot machined into the stator. The two hooks of the mating slot provide rigid support for leaf spring deformation during the motion of the retractable seal towards and away from the rotor. The pressure differential across the seal loads it against the aft support hook so as to form a gas joint that prevents bias leakage through the support hook. Over the operating life of the retractable brush seal, however, the coefficient of friction at a seal/hook contact surface may vary due to oxidation and corrosion of the mating parts as well as due to surface finish changes resulting from fretting or sliding wear between the parts. As a result, seal closure and retraction behavior may vary over time. Such changes may result in the seal not closing fully to the desired low clearance positioning during startup or not retracting before the thermal interference between the stator and rotor occurs during shutdown. The former may result in a significant performance reduction while the later may result in excessive seal wear or damage.
There is thus a desire for an improved retractable seal system that eliminates or reduces the impact of frictional forces on seal motion. Such an improvement should provide overall seal system predictability, reliability, and increased lifetime. Given such, overall leakage performance may be improved over the long term for increased overall turbine engine efficiency and power output.